feat(google-antigravity-auth): create auth plugin for Google models

- Implement createGoogleAntigravityAuthPlugin factory function
- Add OAuth method with PKCE for Google authentication
- Create custom fetch interceptor loader for Antigravity API
- Update auth.ts to export Google Antigravity plugin as default
- Update barrel export in antigravity/index.ts
- Add Google Antigravity auth location to AGENTS.md

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src/auth/antigravity/index.ts

@@ -10,3 +10,4 @@ export * from "./response"
 export * from "./tools"
 export * from "./thinking"
 export * from "./fetch"
+export * from "./plugin"